1. Field of the Invention
The present invention relates to a distance measuring device of a camera for accurately performing a distance measuring operation by correcting an error in distance measurement in accordance with the intensity of external light in a distance measuring system of a light emitting type.
2. Description of the Related Art
There is a double image conforming system in a passive system using external light as a distance measuring system of an automatic focusing adjustment in a so-called compact camera, etc.
A movable mirror for changing a relative position of one image with respect to another image is indispensable to this double image conforming system in the passive system. Accordingly, durability of the camera is reduced by using this movable mirror. Further, a distance measuring operation is performed by the contrast information of a photographed object to be measured since the double image conforming system is used. Accordingly, the distance measuring operation greatly depends on the photographed object. Therefore, distance measuring ability is reduced when the photographed object has a weak contrast and it is dark. Further, such a double image conforming system having a movable section has disadvantages that it is complicated to adjust an operation of the double image conforming system and it takes much time and labor to adjust this operation.
No distance measuring operation greatly depends on the photographed object in the case of a trigonometrical survey system using an active system for emitting light, etc. from a distance measuring device. However, there are also problems about the reduction of durability of the camera and the complicated adjustment mentioned above when this trigonometrical survey system has a movable section for rotating a light emitting portion or a light receiving portion of infrared rays. etc.
There is a supersonic wave system as one kind of the active system. In this supersonic wave system, a supersonic wave is emitted from a supersonic wave emitting portion to a measured object and is reflected on the measured object. A distance between the supersonic wave emitting portion and the measured object is measured from a time required from the emission of the supersonic wave to the reception thereof. In this supersonic wave system, a distance measuring operation is easily performed since the above distance is measured by only electric processing. However, it is necessary to emit the supersonic wave with high power so that a large-sized power source is required. For example, it is difficult to emit an effective supersonic wave by using a power source used in a compact camera, etc.
It is necessary to improve directivity of the supersonic wave so as to prevent an accuracy in distance measurement from being reduced by emitting the supersonic wave to an object except for the measured object. Therefore, it is necessary to increase an are for each of faces for emitting and receiving the supersonic wave. Such requirements cause a big problem of the compact camera, etc.
In a distance detector described next, no high power is required and it is easy to adjust an operation of the camera. Further, it is possible to provide a preferable durability of the camera and a high distance resolution.
This distance detector has a light source for emitting pulse light to a measured object and a semiconductor optical position detector called a PSD in the following description. The semiconductor optical position detector is disposed in a position for focusing and forming a spot of the pulse light reflected on the measured object as an image. The position of an incident spot according to a distance of the measured object based on a parallax between the light source and the semiconductor optical position detector is continuously detected by the semiconductor optical position detector in a position-changing direction in which this distance is changed. Thus, the semiconductor optical position detector provides first and second electric current outputs having a mutual electric current ratio according to the detected position of the incident spot.
The distance detector also has a first detecting circuit for receiving the first electric current output of the semiconductor optical position detector and logarithmically transforming and outputting a changing amount of the first electric current output provided by the pulse light. The distance detector also has a second detecting circuit for receiving the second electric current output of the semiconductor optical position detector and logarithmically transforming and outputting a changing amount of the second electric current output provided by the pulse light. The distance detector further has a differential detecting circuit for providing a distance detecting signal by calculating a difference between the changing amounts of the first and second electric current outputs logarithmically transformed and outputted by the first and second detecting circuits.
In this distance detector, there is no problem about a distance measurement in a dark place. However, at the time of a general photographing operation, etc., there is stationary light having a quantity much larger than that of th pulse light from a pulse light emitting device so that no reflected light can be extracted from the pulse light.
Therefore, in this case, an influence of the stationary light is removed from the pulse light by the first detecting circuit and the second detecting circuit for respectively receiving the first and second electric current outputs of the PSD. Changing amounts of photoelectric currents provided by only the reflected light of the pulse light are respectively transformed logarithmically and outputted from the first and second detecting circuits. The differential detecting circuit calculates a difference between these changing amounts and outputs a distance detecting signal corresponding to an electric current ratio of the first and second electric current outputs of the PSD.
However, such a distance detector has a problem about an operation thereof when a quantity of the stationary light is increased.
Therefore, the inventors of the present application proposed a distance detector having the following structure for solving such a problem in Japanese Patent Application Laying Open (KOKAI) No. 57-192815.
This distance detector has first and second detecting circuits for logarithmically transforming only a changing amount of the output of a semiconductor optical position detector (PSD) provided by pulse light. The distance detector also has a first transistor for logarithmic compression having a collector for receiving an input current signal of each of the first and second detecting circuits. The distance detector also has a first operational amplifier having an inverted input terminal connected to the collector of the first transistor and an output terminal connected to an emitter of the first transistor. The distance detector also has a second transistor having an emitter connected to the output terminal of the first operational amplifier. The distance detector also has a second operational amplifier having an inverted input terminal connected to a collector of the second transistor.
The distance detector also has an electric current supplying circuit for setting an output voltage of the second operational amplifier to a control voltage and supplying an electric current to the collector of the second transistor in accordance with this control voltage. The distance detector also has a capacitor for holding the control voltage in this electric current supplying circuit. The distance detector also has a diode or a third transistor. The diode or the third transistor supplies an electric current corresponding to a changing amount of the above input electric current signal from an output of the second operational amplifier to the collector of the second transistor only at a receiving time of the pulse light emitted from a light source. A voltage drop corresponding to a logarithmic value of this corresponding electric current is caused in the diode or the third transistor. In the distance detector, a detecting output is provided from an output terminal of the second operational amplifier. Such a structure of the distance detector is provided to remove the above problems about transistor characteristics and stabilize an operation of the distance detector.
In such a distance detector, two analog switches are disposed to switch electric circuits in accordance with a stationary state and a pulse light emitting state. A detecting signal with respect to the pulse light is extracted by this switching operation. Therefore, it is not easy to operate the analog switches at a suitable timing. In particular, when the above structure is replaced with integrated circuits (IC), it is difficult to operate the two analog switches at a suitable timing. Further, oscillation tends to be caused so that no distance detector is stably operated. Therefore, no electric circuits of the distance detector can be stably operated so that it is difficult to construct the distance detector by integrated circuits.
In consideration of such a situation, the same applicant as this patent application filed an application about a distance detector with the Japanese Patent Office. This application is already published as Japanese Patent Publication (KOKOKU) No. 2-24325. In this distance detector, the unstable elements of electric circuits are removed by using a relatively simple circuit structure. Accordingly, the distance detector can be stably operated sufficiently when the distance detector is constructed by integrated circuits.
This distance detector has first and second detecting circuits for logarithmically transforming only a changing amount of the output of a semiconductor optical position detector (PSD) provided by pulse light. The distance detector also has a first transistor having an emitter for receiving an input current signal of each of the first and second detecting circuits. The distance detector also has a first operational amplifier having an inverted input terminal connected to the emitter of the first transistor and an output terminal connected to a base of the first transistor. The distance detector also has a second transistor having a collector or a drain connected to a collector of the first transistor. The distance detector also has a second operational amplifier having an non-inverted input terminal connected to the collector or the drain of the second transistor and an output terminal connected to a base or a gate of the second transistor. The second operational amplifier has an inverted input terminal having a predetermined voltage.
The distance detector also has a third transistor having a base connected to the collector or the drain of the second transistor. The distance detector also has a capacitor connected between the base (or the gate) and an emitter (or a source) of the second transistor and having a sufficient capacity. The distance detector further has an electric current detecting circuit for detecting a collector current of the third transistor and providing this collector current to a differential detecting circuit. In this distance detector, only a signal component with respect to the pulse light is extracted without switching analog switches.
However, in this distance detector described in the Japanese patent publication, it is difficult to prevent an error in distance measurement from being caused by external light. Reflected light of signal light emitted from a light emitting element has a constant intensity at the same distance of the photographed object. In contrast to this, an intensity of the external light is greatly changed in accordance with brightness of light around the distance detector. When the intensity of the external light is large, an S/N ratio in a light receiving circuit is reduced so that distance measuring results are changed. Namely, it is difficult to accurately measure the distance between the photographed object and the camera.